The present invention relates to a process for dehydrating and fractionating a low-pressure natural gas.
French patent FR-B-2,605,241 describes a process for treating a natural gas or a refinery gas containing water so as to eliminate at least part of the water contained in said gas, wherein:
a) the gas is contacted with a recycled liquid phase containing water and a solvent (methanol for example); the resulting aqueous liquid phase is discharged and a gas containing, in the vapour state, water and the major part of the solvent is recovered,
b) said gas is cooled so as to condense a liquid phase essentially consisting of water and solvent, and
c) the non-condensed gas is separated from this liquid phase, which forms the phase recycled to stage (a).
French patent FR-B-2,787,870 notably describes a process for fractionating a high-pressure natural gas (pressure above about 5 MPa) wherein at least part of the gas is expanded so as to be used as a cooling agent, expansion being carried out before the fractionating operation, and for gas purification (scrubbing), this operation being carried out by means of a technique allowing simultaneously distillation and heat exchange (for example in an exchanger-dephlegmator). This prior document also describes integration of the fractionating process with a dehydration process using methanol.
Now, there are cases where the natural gas, after coming out of the well or passing through various installations or equipments, is at a pressure lower than about 5 MPa. To fractionate these gases (i.e. extract the condensates or NGL), a new process has been discovered which also uses a simultaneous heat exchange and distillation operation (for example in an exchanger-dephlegmator type device) which, integrated with a dehydration stage, affords the advantage of being very simple and of requiring reduced investment costs.
The invention thus proposes a new process allowing both dehydration and fractionation (stripping) of a low-pressure wet natural gas containing notably hydrocarbon constituents referred to as  less than  less than heavy greater than  greater than  constituents (C3+), hydrocarbon constituents referred to as  less than  less than light  greater than  greater than  constituents (C1 and C2) and water.
The process of the invention, applied to a low-pressure (P0 below about 5 MPa) we natural gas containing constituents referred to as  less than  less than heavy  greater than  greater than  constituents and constituents referred to as  less than  less than light  greater than  greater than  constituents, can generally be defined by the fact that it comprises in combination at least the following stages:
a) at least a fraction of the wet gas at a temperature T0 is contacted with an aqueous liquid phase L""1 containing methanol, said gas carrying along substantially all of the methanol contained in said aqueous phase L""1,
b) the gas from stage (a) is cooled to a temperature T1 lower than temperature T0, which produces a gas phase G1 at equilibrium with a hydrocarbon-containing liquid phase L1 containing C3+ and an aqueous liquid phase L""1 containing methanol,
c) aqueous liquid phase L""1 is sent to stage (a), and
d) gas phase G1 is fractionated by distillation carried out by continuous thermal exchange with a cooling fluid, which allows to extract, on the one hand, the  less than  less than light) greater than  greater than  constituents in the gas form (phase G2) and, on the other hand, the  less than  less than heavy greater than  greater than  constituents in the form of condensates (phase L2).
To carry out the thermal exchange in stage (d), it is possible to use an external cooling fluid such as a propane cycle. However, according to a preferred embodiment of the process of the invention, the cooling fluid consists of gas G2 itself after expansion. In this case, the expanded gas is heated by the thermal exchange of stage (d); it can also be heated by thermal exchange with the gas entering stage (b). It is recompressed thereafter in order to be exported.
More particularly, refrigeration of the gas in stage (b) can be achieved successively with cooling water or an air-cooled exchanger, then by exchange with the cooling fluid of stage (d), and finally, if necessary, by means of an external coolant such as a propane cycle.
Preferred and advantageous embodiments of the process according to the invention are described hereafter.
The hydrocarbon liquid phase L2 formed in stage (d) and comprising mainly C3+ condensates can also contain lighter hydrocarbons (C1 and C2) and require stabilization. It can then be sent to a stabilization stage (e) in order to obtain stabilized condensates L3 and a gas phase G3, to be fractionated, which is then sent back to fractionating stage (d).
Small amounts of methanol which it may be desirable to recover generally remain in the C3+ condensates from stage (d) (phase L2) or in the stabilized C3+ condensates from stabilization stage (e) (phase L3). A condensate washing stage (f) can therefore be carried out using an aqueous phase; the aqueous phase having thus extracted the methanol can then be partly regenerated, in a regeneration stage (g), for example by stripping with the remaining fraction of the initial wet natural gas that has not been contacted with the aqueous liquid phase L""1 in stage (a). The aqueous phase thus partly regenerated in regeneration stage (g) is used again to extract the methanol from the C3+ condensates in washing stage (f).
In the latter case, after stripping of the methanol, the natural gas fraction considered is mixed with the fraction that has been contacted with the aqueous liquid phase L""1 in stage (a).
During the fractionating stage of the process according to the invention, methanol losses can occur. To compensate for these losses, extra methanol can be added, for example in the gas upstream from cooling stage (b) and/or at the start of the distillation operation of stage (d) and/or during the stage of expansion of gas G2.